1. Field of the Invention
The present invention relates generally to multiple annular combustors for a gas turbine engine and, in particular, to the manner of retaining a centerbody in position between adjacent domes of such multiple annular combustors.
2. Description of Related Art
Efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for high temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions. The swirl cups of the respective pilot and main stage sections generally lie in the same radial and circumferential planes, as exemplified by U.S. Pat. No. 4,292,801 to Wilkes et al. and U.S. Pat. Nos. 4,374,466 and 4,249,373 to Sotheran.
However, as discussed in a development report to the National Aeronautics and Space Administration (NASA) on combustion system component technology for the Energy Efficient Engine (E.sup.3) and U.S. Pat. No. 4,194,358 to Stenger, the pilot stage and the main stage may be radially offset (i.e., lie in distinct radial planes). In both the '358 patent and E.sup.3 configurations, the effective length of the main stage section is relatively short and the effective length of the pilot stage section is relatively long.
This configuration allows for complete or near-complete combustion to reduce the amount of hydrocarbon and carbon monoxide emissions since there is a relatively long residence time in the pilot stage section and a relatively minimal residence time in the main stage section.
Whether the inner and outer combustors are radially aligned or not, and whether the outer annular combustor acts as the pilot stage or main stage, the prior art discloses the use of a centerbody between the pilot and main stages. The intended purpose of such centerbodies is to isolate the pilot stage from the main stage in order to ensure combustion stability of the pilot stage at various operating points and to allow primary dilution air to be directed into the pilot stage reaction zone.
Until recently, such centerbodies have been a continuous ring fabricated from forged or rolled rings and sheet material. Such one-piece designs were difficult to manufacture due to tight size and form tolerance requirements for fabrication and assembly. Moreover, the difference in temperature between the combustor structure and the centerbody generated large hoop stresses and associated forces at the point of attachment. In order solve these and other problems stemming from one-piece centerbody designs, a centerbody has been developed which is made up of a plurality of independent arcuate segments which are connected to either the inner or outer domes of the combustor (see U.S. Pat. No. 5,375,420 to Falls et al.).
Centerbodies in general and centerbody segments in particular have previously been attached to the inner and/or outer domes of the combustor through a bolted connection or brazing. Since the centerbody is located in a hostile environment in which the flame temperatures approach ideal stoichiometric reaction (4000.degree. F.), the life of this component is limited due to the eventual oxidation of the metal (despite cooling air and thermal barrier coatings used to protect the parent metal from the extreme temperatures). Because the prior methods of attaching the centerbody segments to the combustor have made it difficult to replace such segments in the field, it would be desirable if a new manner of attaching them would be developed that would allow the engine user to more easily maintain the combustor. Accordingly, the present invention provides a new way of retaining the centerbody segments in position between the inner and outer domes of a combustor which facilitates the insertion and removal thereof.